Hitherto, a solid-state image pickup device, such as a CMOS image sensor and a CCD image sensor, has had a construction including an imaging pixel unit having multiple pixels in a two-dimensional array form, for example, which are photoelectric converters (photodiodes), on a semiconductor substrate, a wiring layer in a multi-layer structure thereon, which has different signal wires and a light-shield film, and an on-chip color filter and/or an on-chip microlens thereon through a passivation layer.
Recently, in particular, with increases in the number of pixels (that is, high-density) and the functionality of the imaging pixel unit, the number of layers in the wiring layer and the complexity of a layout pattern thereof are increased. Because of an increase in the thickness thereof, a distance between an optical system, such as a color filter and a microlens, and a photodiode-receptive plane (called sensor photoreceptive portion, hereinafter) tends to increase.
Moreover, because of the microstructure due to an increase in the number of pixels of the imaging pixel unit, the lens form of the microlens tends to be minute.
However, as the distance between the photodiode and the microlens increases and/or the size of the microlens decreases, the angle of incidence of a main light beam differs in the center and peripheral parts of the screen of the imaging pixel unit, and the depth of the semiconductor substrate on which incident light is photoelectrically converted changes, which disadvantageously changes the amount of shading in accordance with the wavelength of the incident light. Since the amount of received light of the sensor photoreceptive portion differs in the center and peripheral parts of the screen, keeping even sensitivity therein may be difficult.
In particular, as the area for each pixel decreases and the size of the lens decreases, the angle of incidence of light on the peripheral part of the screen increases. On the other hand, in a pixel reduced in size, the form of the sensor photoreceptive portion becomes vertical and/or horizontal at the asymmetrical screen as described later, and the top and bottom and left and right ends of the screen may have uneven sensitivities.
Furthermore, in a CMOS image sensor having multiple wires such as a readout gate, a signal wire and a power-supply wire in an area near the photodiodes thereof, an effective pixel area becomes asymmetrical more easily, and the above-described problem becomes more significant.
In order to solve the problem, the sensitivity of the screen peripheral part is conventionally designed to increase by shifting a position of each lens and/or a position of an opening of the light-shield film in accordance with a position of a pixel thereof in the imaging pixel unit.
For example, FIG. 7 is a plan view showing a positional relationship among each pixel, the light-shield film opening part and collective lens in the imaging pixel unit of such a conventional solid-state image pickup device, and FIGS. 8A and 8B are explanatory diagrams illustrating decreases in sensitivity at the diagonal lines A-A′ and B-B′ of the imaging pixel unit shown in FIG. 7.
In FIG. 7, the light-shield film opening part and a collective lens 11 are placed in a pixel 10 in a center part of the screen at a position having the center agreeing with the center of the pixel 10 and are placed in a pixel 10 in the peripheral part of the screen at positions shifted in the direction of the incidence of light.
Furthermore, a readout gate portion 10B for reading out signal charges of a sensor photoreceptive portion 10A is provided at the lower left corner of the sensor photoreceptive portion 10A of each of the pixels 10.
Thus, the form of the sensor photoreceptive portion 10A of each of the pixels 10 is vertically and/or horizontally asymmetrical from the center of the screen due to the presence of the readout gate portion 10B, and the presence of the readout gate portion 10B more largely affects the asymmetry as the area of the pixel 10 decreases.
Therefore, even in a solid-state image pickup device having a light-shield film opening part and collective lens thereof at shifted positions, the asymmetry of each of the pixels 10 significantly decreases the sensitivity in the pixels 10 near the point A as shown in FIG. 8A, for example, and the sensitivity varies unevenly in the vertical and/or horizontal direction(s).
As a proposal for optimizing the amount of incident light on each pixel, one is known in which the positions of a lens, filter, light-shield film, sensor photoreceptive portion and so on are corrected in accordance with the distance from the center of an imaging area of each pixel and the height from a photoreceptive surface of the lens, as disclosed in JP-A-5-328233, JP-A-2000-349268, JP-A-2001-160973, JP-A-2001-210812 and so on. However, even with the proposal, effectively addressing the asymmetry of the sensor photoreceptive portion as described above is difficult. This may be because the position in the photoreceptive surface that the distance from the center of the imaging area to the photoreceptive portion covers is not studied and because the fact that light gathered by a lens launches on the surface of the photoreceptive portion is only considered though the positions of components in a pixel are corrected but the thickness in the direction of depth of a photoelectric converting portion in which photoelectric conversion actually occurs is not considered.
Accordingly, it is an object of the invention to provide a solid-state image pickup device and a method of manufacturing the same, which can obtain a proper incident state in each pixel and which can achieve an improvement in photoreceptive efficiency and even sensitivity in each pixel.